Such inhalers, generally referred to as Breath-Actuated Inhalers (BAIs) and using metering valve systems (MDI systems), are usually based on a trigger mechanism having a spring, said spring being released by a suitable device when the user inhales. Loading or compressing the spring is usually achieved by actuating a lever, e.g. when opening the lid on the mouthpiece of the inhaler, and, on inhaling, the force from the spring is directed against the reservoir or against the metering valve, enabling the valve to be actuated by moving the valve member relative to the reservoir. This is made possible by the fact that one of the elements comprising the valve member and the reservoir which is not subjected to the drive from the pre-cocked spring, is held stationary inside the device. As a result, after it has been actuated, and after the fluid contained in the reservoir has been dispensed, the metering valve remains compressed with the valve member in its actuating position until the load on the spring is released, which occurs only once the lid of the mouthpiece has been closed again.
The above-described structure is the source of a problem which is related to the manner in which most metering valves operate. Such a valve generally has a return spring and a metering chamber which is filled with a mixture made up of the fluid, in general medication, and of the liquefied propellant gas. The metering chamber is filled under gravity and only when the valve member moves from its dispensing position to its rest position, i.e. when the force applied on the valve by the spring of the trigger system is released. This therefore requires the tension of the spring to be released when the device is in a position suitable for enabling the metering chamber of the valve to be filled by gravity. The position required in order for the metering chamber to be filled effectively and fully is the in-use position of the inhaler, in which position the reservoir is generally disposed above the metering valve, the user having the mouthpiece in the mouth for the purpose of inhaling the metered quantity or “dose” of fluid dispensed.
As of the end of dispensing of the dose of fluid, when the user withdraws the device from the mouth, there is a high probability that the inhaler is no longer in the position required for effective filling, and there is a high risk that the user might close the lid of the mouthpiece while the inhaler device is in a position that is unsuitable for filling the metering chamber fully.